Multi-flute ball end mill

Abstract

A multi-flute ball end mill comprising a shank portion, a cutting edge portion having a ball-shaped tip portion, and 3 or more cutting edges formed in the cutting edge portion; each cutting edge being constituted by a peripheral cutting edge having a twist angle η of 35-45°, and a ball-end cutting edge having a twist angle μ at the outermost peripheral point, the twist angle η and the twist angle μ meeting the relation of η−μ≤7°, such that the ball-end cutting edge is smoothly connected to the peripheral cutting edge; the ball-end cutting edge having a radial rake angle of −37° to −11°; the peripheral cutting edge having a radial rake angle of 2-8°; and a center-lowered, inclined cutting edge integrally extending from a tip end of each ball-end cutting edge to a rotation center point, in a tip end portion of the ball portion near the rotation center point.

What is claimed is: 
     
       1. A multi-flute ball end mill comprising a shank portion rotating around a rotation axis, a cutting edge portion having a ball-shaped tip portion, and 3 or more cutting edges formed in said cutting edge portion;
 each cutting edge being constituted by a peripheral cutting edge having a twist angle η of 35-45°, and a ball-end cutting edge having a twist angle μ at the outermost peripheral point, said twist angle η and said twist angle μ meeting the relation of η−μ≤7°, such that said ball-end cutting edge is smoothly connected to said peripheral cutting edge; 
 said ball-end cutting edge having a radial rake angle of −37° to −11°, and said peripheral cutting edge having a radial rake angle of 2-8°, in a range from 0.1D to 0.4D from a connecting point of each center-lowered, inclined cutting edge and each ball-end cutting edge, wherein D is a diameter of said cutting edge portion; 
 said center-lowered, inclined cutting edge integrally extending from a tip end of each ball-end cutting edge to a rotation center point, in a tip end portion of said ball portion near said rotation center point; and 
 a second concave rake face having a positive rake angle, which corresponds to a rake face of each peripheral cutting edge, entering a center portion of a first rake face having a negative rake angle, which corresponds to a rake face of each ball-end cutting edge, in a region of each ball-end cutting edge near a boundary of said ball-end cutting edge and said peripheral cutting edge, a ratio of said second rake face to said first rake face gradually increasing as nearing said boundary, with said second rake face reaching 100% at said boundary. 
 
     
     
       2. The multi-flute ball end mill according to  claim 1 , wherein each center-lowered, inclined cutting edge has at least an arcuate portion projecting rearward in a rotation direction, said arcuate portion having a curvature (ratio of the length of a perpendicular line extending from an apex of said arcuate portion to a line connecting both ends of said arcuate portion to the length of said line connecting both ends of said arcuate portion) of 5-40%; and wherein each center-lowered, inclined cutting edge is inclined with an inclination angle α of 0.5-3° relative to a plane perpendicular to said rotation axis, such that said rotation center point is positioned on the rear side of a connecting point of each center-lowered, inclined cutting edge and each ball-end cutting edge in a rotation axis direction. 
     
     
       3. The multi-flute ball end mill according to  claim 1 , wherein a boundary of said first rake face and said second rake face is in a curved shape projecting toward a tip end of each ball-end cutting edge. 
     
     
       4. The multi-flute ball end mill according to  claim 1 , wherein a chip-discharging groove between said ball-end cutting edges has a convex bottom surface; and wherein the curvature of said convex bottom surface (ratio of the length of a perpendicular line extending from an apex of said convex bottom surface to a line connecting both ends of said convex bottom surface to the length of said line connecting both ends of said convex bottom surface) is 5-40%. 
     
     
       5. The multi-flute ball end mill according to  claim 4 , wherein 50% or more of a bottom surface of said chip-discharging groove is occupied by said convex surface portion. 
     
     
       6. The multi-flute ball end mill according to  claim 1 , wherein the percentage of the radial length of said arcuate portion in each center-lowered, inclined cutting edge is 20-100%; wherein the circumferential width of a flank of each center-lowered, inclined cutting edge at a connecting point of the center-lowered, inclined cutting edge and the ball-end cutting edge in each cutting edge is 20-80% of the maximum circumferential width of a flank of each ball-end cutting edge; and wherein the radial length X of each center-lowered, inclined cutting edge (radial distance between an outer end of said ball-end cutting edge and said rotation center point) is 1.25-3.75% of the diameter D of said cutting edge portion. 
     
     
       7. The multi-flute ball end mill according to  claim 1 , wherein said ball-end cutting edges are unevenly arranged circumferentially around said rotation axis. 
     
     
       8. A multi-flute ball end mill comprising a shank portion rotating around a rotation axis, a cutting edge portion having a ball-shaped tip portion, and 3 or more cutting edges formed in said cutting edge portion;
 each cutting edge being constituted by a peripheral cutting edge having a twist angle η of 35-45°, and a ball-end cutting edge having a twist angle μ at the outermost peripheral point, said twist angle η and said twist angle μ meeting the relation of η−μ≤7°, such that said ball-end cutting edge is smoothly connected to said peripheral cutting edge; 
 said ball-end cutting edge having a radial rake angle of −37° to −11°, and said peripheral cutting edge having a radial rake angle of 2-8°, in a range from 0.1D to 0.4D from a connecting point of each center-lowered, inclined cutting edge and each ball-end cutting edge, wherein D is a diameter of said cutting edge portion; 
 said center-lowered, inclined cutting edge integrally extending from a tip end of each ball-end cutting edge to a rotation center point, in a tip end portion of said ball portion near said rotation center point; and 
 a rake face of each ball-end cutting edge being a curved surface convexly projecting forward in a rotation direction, the curvature of said convexly curved surface (ratio of the length of a perpendicular line extending from an apex of said convexly curved surface to a line connecting both ends of said convexly curved surface to the length of said line connecting both ends of said convexly curved surface) being 1-10%. 
 
     
     
       9. The multi-flute ball end mill according to  claim 8 , wherein each center-lowered, inclined cutting edge has at least an arcuate portion projecting rearward in a rotation direction, said arcuate portion having a curvature (ratio of the length of a perpendicular line extending from an apex said arcuate portion to a line connecting both ends of said arcuate portion to the length of said line connecting both ends of said arcuate portion) of 5-40%; and wherein each center-lowered, inclined cutting edge is inclined with an inclination angle α of 0.5-3° relative to a plane perpendicular to said rotation axis, such that said rotation center point is positioned on the rear side of a connecting point of each center-lowered, inclined cutting edge and each ball-end cutting edge in a rotation axis direction. 
     
     
       10. The multi-flute ball end mill according to  claim 8 , wherein a chip-discharging groove between said ball-end cutting edges has a convex bottom surface; and wherein the curvature of said convex bottom surface (ratio of the length of a perpendicular line extending from an apex of said convex bottom surface to a line connecting both ends of said convex bottom surface to the length of said line connecting both ends of said convex bottom surface) is 5-40%. 
     
     
       11. The multi-flute ball end mill according to  claim 10 , wherein 50% or more of a bottom surface of said chip-discharging groove is occupied by said convex surface portion. 
     
     
       12. The multi-flute ball end mill according to  claim 8 , wherein the percentage of the radial length of said arcuate portion in each center-lowered, inclined cutting edge is 20-100%; wherein the circumferential width of a flank of each center-lowered, inclined cutting edge at a connecting point of the center-lowered, inclined cutting edge and the ball-end cutting edge in each cutting edge is 20-80% of the maximum circumferential width of a flank of each ball-end cutting edge; and wherein the radial length X of each center-lowered, inclined cutting edge (radial distance between an outer end of said ball-end cutting edge and said rotation center point) is 1.25-3.75% of the diameter D of said cutting edge portion. 
     
     
       13. The multi-flute ball end mill according to  claim 8 , wherein said ball-end cutting edges are unevenly arranged circumferentially around said rotation axis.